U.S. patent number 7,909,772 [Application Number 11/578,462] was granted by the patent office on 2011-03-22 for non-invasive measurement of second heart sound components.
This patent grant is currently assigned to Masimo Corporation. Invention is credited to Rajeev Agarwal, Victor F. Lanzo, Boris Popov.
United States Patent |
7,909,772 |
Popov , et al. |
March 22, 2011 |
Non-invasive measurement of second heart sound components
Abstract
A method and apparatus for estimating a location of pulmonary
and aortic components of second heart sounds of a patient over an
interval. The method comprises the steps of producing an electronic
representation of heart sounds of the patient over the interval,
identifying at least one second heart sound in the interval using
the electronic representation, for each identified second heart
sound generating an estimated value for a location of the aortic
component and the pulmonary component. There is also included a
method for using the estimated location of the aortic component and
the pulmonary component for estimation of the blood pressure in the
pulmonary artery of a patient.
Inventors: |
Popov; Boris (Montreal,
CA), Lanzo; Victor F. (Laval, CA), Agarwal;
Rajeev (Dollard-des-Ormeaux, CA) |
Assignee: |
Masimo Corporation (Irvine,
CA)
|
Family
ID: |
35276856 |
Appl.
No.: |
11/578,462 |
Filed: |
April 15, 2005 |
PCT
Filed: |
April 15, 2005 |
PCT No.: |
PCT/CA2005/000568 |
371(c)(1),(2),(4) Date: |
August 13, 2007 |
PCT
Pub. No.: |
WO2005/099562 |
PCT
Pub. Date: |
October 27, 2005 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20080091115 A1 |
Apr 17, 2008 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
60562538 |
Apr 16, 2004 |
|
|
|
|
Foreign Application Priority Data
|
|
|
|
|
Apr 16, 2004 [CA] |
|
|
2464634 |
|
Current U.S.
Class: |
600/528; 600/586;
600/485 |
Current CPC
Class: |
A61B
7/04 (20130101); A61B 5/021 (20130101); A61B
5/352 (20210101); A61B 5/369 (20210101) |
Current International
Class: |
A61B
5/02 (20060101) |
Field of
Search: |
;600/528,485 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Adaptive Segmentation of Electroencephalographic Data using
Non-Linear Energy Operator (Agarwal et al., Proceedings of IEEE
'99, ISCAS'99, Orlando, Florida, 1999). cited by other.
|
Primary Examiner: Layno; Carl H.
Assistant Examiner: Morales; Jon-Eric C.
Attorney, Agent or Firm: Law Office of Glenn R. Smith
Parent Case Text
RELATED APPLICATIONS
This application is the national stage of International Application
No. PCT/CA2005/000568, filed Apr. 15, 2005, which claims the
benefit of and priority from U.S. Provisional No. 60/562,538, filed
Apr. 16, 2004, which are incorporated by reference herein.
Claims
What is claimed is:
1. A method for estimating a location of pulmonary and aortic
components of second heart sounds of a patient over an interval,
the method comprising the steps of: producing an electronic
representation of heart sounds of the patient over the interval;
identifying at least one second heart sound in the interval using
said electronic representation; for each identified second heart
sound: calculating a frequency weighted energy (FWE); normalizing
said FWE; identifying peaks in said FWE; determining a maximum peak
from said identified peaks; and retaining said maximum peak and
peaks having an amplitude within a predetermined amount of an
amplitude of said maximum peak; wherein if two or more peaks are
retained, two largest peaks are selected, a first peak as a
candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein said first
peak is prior to said second peak and wherein if only a single peak
is retained, said single peak is selected as a candidate value for
the aortic component; and generating an estimated value for a
location of the aortic component and the pulmonary component from
said candidate values; wherein said FWE calculating step comprises
generating a Non Linear Energy Operator (NLEO) function for each
identified second heart sound; and wherein in said peak identifying
step, a maximum peak value is identified and any identified peaks
having a value less than a predetermined amount of said maximum
peak value are discarded.
2. A method for estimating a location of pulmonary and aortic
components of second heart sounds of a patient over an interval,
the method comprising the steps of: producing an electronic
representation of heart sounds of the patient over the interval;
identifying at least one second heart sound in the interval using
said electronic representation; for each identified second heart
sound: calculating a frequency weighted energy (EWE); normalizing
said FWE; identifying peaks in said FWE; determining a maximum peak
from said identified peaks; and retaining said maximum peak and
peaks having an amplitude within a predetermined amount of an
amplitude of said maximum peak; wherein if two or more peaks are
retained, two largest peaks are selected, a first peak as a
candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein said first
peak is prior to said second peak and wherein if only a single peak
is retained, said single Leak is selected as a candidate value for
the aortic component; generating an estimated value for a location
of the aortic component and the pulmonary component from said
candidate values; for each identified second heart sound and prior
to said FWE calculating step, a signal to noise ratio (SNR)
calculating step wherein when said calculated SNR is below a
predetermined ratio, no candidate values for the aortic component
or the pulmonary component are selected; wherein said SNR
calculating step comprises detecting a start and finish of said
identified second heart sound, calculating an average signal energy
between said start and finish, calculating an average noise energy
for a predetermined period before said start and after said finish,
wherein said SNR is equal to said average signal energy divided by
said average noise energy; and wherein said predetermined period is
50 ms.
3. A method for estimating a location of pulmonary and aortic
components of second heart sounds of a patient over an interval,
the method comprising the steps of: producing an electronic
representation of heart sounds of the patient over the interval;
identifying at least one second heart sound in the interval using
said electronic representation; for each identified second heart
sound: calculating a frequency weighted energy (FWE); normalizing
said FWE; identifying peaks in said FWE; determining a maximum peak
from said identified peaks; and retaining said maximum peak and
peaks having an amplitude within a predetermined amount of an
amplitude of said maximum peak; wherein if two or more peaks are
retained, two largest peaks are selected, a first peak as a
candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein said first
peak is prior to said second peak and wherein if only a single peak
is retained, said single peak is selected as a candidate value for
the aortic component; generating an estimated value for a location
of the aortic component and the pulmonary component from said
candidate values; for each identified second heart sound and prior
to said FWE calculating step, a signal to noise ratio (SNR)
calculating step wherein when said calculated SNR is below a
predetermined ratio, no candidate values for the aortic component
or the pulmonary component are selected; and wherein said
predetermined ratio is 0.50.
4. A method for estimating pulmonary artery pressure of a patient
over an interval, the method comprising the steps of: producing an
electronic representation of heart sounds of the patient over the
interval; identifying at least one second heart sound in the
interval using said electronic representation; for each identified
second heart sound: calculating a FWE; normalizing said FWE;
identifying peaks in said FWE; determining a maximum peak from said
identified peaks; and retaining said maximum peak and peaks having
an amplitude within a predetermined amount of an amplitude of said
maximum peak; wherein if two or more peaks are retained, two
largest peaks are selected, a first peak as a candidate value for
the aortic component and a second peak as a candidate value for the
pulmonary component, wherein said first peak is prior to said
second peak and wherein if only a single peak is retained, said
single peak is selected as a candidate value for the aortic
component; and generating an estimated value for a location of an
aortic component and a location of pulmonary component from said
candidate values; determining a splitting interval as a time
between said aortic component location and said pulmonary component
location; normalizing said splitting interval; and estimating the
systolic pulmonary artery pressure using a predetermined function
which describes a relationship between said normalized splitting
interval and the systolic and diastolic pulmonary artery
pressures.
5. A device for estimating a location of pulmonary and aortic
components of second heart sounds of a patient over an interval,
the device comprising: at least one transducer for sensing heart
sounds of the patient, said heart sounds comprising, during motion
of the patient, a first sound portion indicative of second heart
sounds and a second sound portion indicative of motion induced
noise; and a second heart sound processor to identify second heart
sounds from said sensed heart sounds and compute an aortic
component location and an estimated pulmonary component location
from said identified second heart sounds without significant
interference in said identification and computation from the
motioned induced noise portion of said sensed heart sounds; wherein
said second heart sound processor comprises: a means for
identifying at least one second heart sound in the interval using
said sensed heart sounds; a means for calculating a frequency
weighted energy (FWE) of each identified second heart sound; a
means for normalizing said calculated FWE of each identified second
heart sound; a means for identifying peaks in said calculated FWE
of each identified second heart sound and determining a maximum
peak from said identified peaks; and a means for retaining said
maximum peak and peaks having an amplitude within a predetermined
amount of an amplitude of said maximum peak wherein if two or more
peaks are retained, two largest peaks are selected, a first peak as
a candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein said first
peak is prior to said second peak and wherein if only a single peak
is retained, said single peak is selected as a candidate value for
the aortic component; and a means for generating an estimated value
for a location of the aortic component and the pulmonary component
from said candidate values; wherein said FWE calculating means
generates a Non Linear Energy Operator (NLEO) function for each
identified second heart sound; and wherein said peak identifying
means identifies a maximum peak value and discards any identified
peak having a value less than a predetermined amount of said
maximum peak value.
Description
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for
non-invasive detection of second heart sound (S2) components. In
particular, the present invention relates to a method and apparatus
for estimating a location of the aortic (A2) and pulmonary (P2)
components of S2 relative to the Q marker of a QRS segment of an
Electrocardiogram (ECG).
BACKGROUND OF THE INVENTION
The highly publicized problem of cardio-vascular diseases, an
increased population living excess of 80, and the predominance of
the heart disease as a leading cause of death have increased the
importance of the clinical practioner's ability to recognize
abnormal heart conditions. One of the most powerful instruments for
non-invasive heart diagnostics is auscultation. Traditionally,
auscultation is based on a physician's ability to use a stethoscope
to recognize specific patterns and phenomena. Through advances in
technology many of these abilities have been automated, however for
some of these auscultation methods a stable automated procedure has
yet to be found.
For diagnostic cardiac events one of the most interesting sounds is
the second heart sound This sound comprises two components which
are generally of interest: the aortic component and the pulmonary
component. Detection and recognition of those components provides
the possibility of measuring the systole and diastole duration for
both the left- and right heart. These values are very important for
many applications such as detection of pulmonary artery
hypertension, dysfunction of heart valves, left and right
ventricular dysfunction, etc.
As described hereinabove, the second heart sound and the components
A2 and P2 thereof have significant clinical value. However, these
components are very often masked by noises and other acoustic
components of both the heart sounds and other parts of human body.
As result, typically only specially trained and experienced
clinicians can distinguish the A2 and P2 components. As a result,
an automated computer-based procedure for A2 and P2 components
would be desirable in clinical practice. One prior art reference,
U.S. Pat. No. 6,368,283, reveals such a method. However, the
proposed method is a non-automated human-assisted procedure which
only works during periods of non-breathing.
Cardiac catheterisation and echocardiography, which have provided
an accurate diagnosis of both right- and left heart abnormalities,
have added a new dimension to usefulness of the phonocardiogram in
assessing the presence and severity of cardiovascular
abnormalities. Although cardiac catheterization generally provides
the decisive evidence of the presence and severity of cardiac
abnormalities, the external sound recordings correlate sufficiently
well with the internal findings for them to serve, in many
instances, as diagnostic tool per se. In this regard,
phonocardiography often provides information complementary to that
obtained by echocardiography. With this enhanced diagnostic
accuracy, simpler and less painful external techniques can be used
to determine when a patient needs more extensive cardiac treatment.
Even in those cases where cardiac catheterisation is deemed
necessary, the knowledge gained beforehand through
phonocardiography and other non-invasive studies can lead to much
more efficient and fruitful invasive study.
SUMMARY OF THE INVENTION
To address the above and other drawbacks, there is provided a
method for estimating a location of pulmonary and aortic components
of second heart sounds of a patient over an interval. The method
comprises the steps of producing an electronic representation of
heart sounds of the patient over the interval, identifying at least
one second heart sound in the interval using the electronic
representation, for each identified second heart sound calculating
a frequency weighted energy (FWE), normalising the FWE, identifying
peaks in the FWE, determining a maximum peak from the identified
peaks and retaining the maximum peak and peaks having an amplitude
within a predetermined amount of an amplitude of the maximum peak,
wherein if two or more peaks are retained, two largest peaks are
selected, a first peak as a candidate value for the aortic
component and a second peak as a candidate value for the pulmonary
component, wherein the first peak is prior to the second peak and
wherein if only a single peak is retained, the single peak is
selected as a candidate value for the aortic component, and
generating an estimated value for a location of the aortic
component and the pulmonary component from the candidate
values.
There is also provided a method for estimating a location of
pulmonary and aortic components of second heart sounds of a patient
over an interval. The method comprises the steps of producing an
electronic representation of heart sounds of the patient over the
interval, dividing the electronic representation into a plurality
of sub-channels, for each of the sub-channel representations,
identifying at least one second heart sound in the interval using
the electronic representation and extracting an estimated location
of a sub-channel aortic component and a sub-channel pulmonary
component from the at least one second heart sound, combining the
estimated sub-channel aortic component locations to form the
estimated aortic component location and the estimated sub-channel
pulmonary component locations to form the estimated pulmonary
component location.
Additionally, there is provided a method for estimating a location
of pulmonary and aortic components of second heart sounds a patient
over an interval. The method comprises the steps of positioning a
first transducer at a first position on the patient, the first
transducer producing a first electronic representation of heart
sounds of the patient over the interval, positioning a second
transducer at a second position on the patient, the second
transducer producing a second electronic representation of heart
sounds of the patient over the interval, for the first electronic
representation identifying at least one second heart sound in the
interval, for each identified second heart sound calculating a FWE,
normalising the FWE, identifying peaks in the FWE, determining a
maximum peak from the identified peaks and retaining the maximum
peak and peaks having an amplitude within a predetermined amount of
an amplitude of the maximum peak, wherein if two or more peaks are
retained, two largest peaks are selected, a first peak as a
candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein the first peak
is prior to the second peak and wherein if only a single peak is
retained, the single peak is selected as a candidate value for the
aortic component, and generating a first estimated value for a
location of an aortic component and a pulmonary component from the
candidate values and for the second electronic representation
identifying at least one second heart sound in the interval, for
each identified second heart sound calculating a FWE, normalising
the FWE, identifying peaks in the FWE, determining a maximum peak
from the identified peaks and retaining the maximum peak and peaks
having an amplitude within a predetermined amount of an amplitude
of the maximum peak, wherein if two or more peaks are retained, two
largest peaks are selected, a first peak as a candidate value for
the aortic component and a second peak as a candidate value for the
pulmonary component, wherein the first peak is prior to the second
peak and wherein if only a single peak is retained, the single peak
is selected as a candidate value for the aortic component and
generating second estimated values for a location of the aortic
component and the pulmonary component from the candidate values and
combining the first and second estimated aortic location values and
the first and second estimated pulmonary location values wherein
the estimated location of the aortic components is the combined
first and second estimated aortic location values and the estimated
location of the pulmonary components is the combined first and
second estimated pulmonary location values.
Furthermore, there is provided a method for estimating pulmonary
artery pressure of a patient over an interval. The method comprises
the steps of producing an electronic representation of heart sounds
of the patient over the interval, identifying at least one second
heart sound in the interval using the electronic representation,
for each identified second heart sound calculating a FWE,
normalising the FWE, identifying peaks in the FWE, determining a
maximum peak from the identified peaks and retaining the maximum
peak and peaks having an amplitude within a predetermined amount of
an amplitude of the maximum peak, wherein if two or more peaks are
retained, two largest peaks are selected, a first peak as a
candidate value for the aortic component and a second peak as a
candidate value for the pulmonary component, wherein the first peak
is prior to the second peak and wherein if only a single peak is
retained, the single peak is selected as a candidate value for the
aortic component and generating an estimated value for a location
of an aortic component and a location of pulmonary component from
the candidate values, determining a splitting interval as a time
between the aortic component location and the pulmonary component
location, normalising the splitting interval, and estimating the
systolic pulmonary artery pressure using a predetermined function
which describes a relationship between the normalised splitting
interval and the systolic and diastolic pulmonary artery
pressures.
Also, there is provided an apparatus implementing any of the above
methods.
BRIEF DESCRIPTION OF THE DRAWINGS
In the appended drawings:
FIG. 1 discloses an illustrative embodiment of a device according
to an illustrative embodiment of the present invention;
FIG. 2 discloses typical signals detected using an ECG and a pair
of biological sound monitors according to an illustrative
embodiment of the present invention; and
FIGS. 3A and 3B disclose a flow chart of the A2, P2 and SI
detection portion of the device according to an illustrative
embodiment of the present invention.
DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
Referring now to FIG. 1, an illustrative embodiment of a device,
generally referred to using the reference numeral 10, will now be
described. Illustratively, two identical biological sound sensors
12, for example those described in U.S. Pat. No. 6,661,161 are
provided for, although in a given application a single or multiple
sensors may be preferable. In the case of the multiple sensor
schemas, those sensors are placed at different locations on the
patient 14, where we expect to find the maximal intensity of the
aortic component of the second heart sound A2 or the pulmonary
component of the second heart sound P2 or both A2 and P2 signals.
In the illustrated example one sensor 12.sub.1 is positioned at the
apex of heart, where the A2 component of the S2 sound is likely at
its maximal in intensity and P2 component is minimal. A second
sensor 12.sub.2 is placed to maximize the P2 component intensity
(between the 3.sup.rd and 4th left intercostal space). The best
sensor locations are obtained by experimenting with different
positions while observing S2 sound signals, so as to achieve the
maximal signal intensity.
The sensors 12 are attached via appropriate leads as in 16 to a
data acquisition system 18 comprised of an analog to digital
converter 20 and personal computer 22. Data collected by the
sensors 12 is digitised by the analog to digital converter 20,
illustratively using a sampling rate of 2 kHz with 12 bits of
resolution. Additionally, Electrocardiogram (ECG) signals are also
collected via a series of electrodes 24, leads 26 and a second
analog to digital converter 28. Similar to the acoustic data
collected by the biological sound sensors 12, data collected by the
ECG electrodes 24 is digitised by the analog to digital converter
28, illustratively using a sampling rate of 2 kHz with 12 bits of
resolution. As will be seen below, the electrocardiogram is used as
the reference signal to frame the second heart sound (S2).
Referring now to FIG. 2, an ECG reading is displayed along side
readings from first and second biological sound sensors.
Automatic A2 and P2 Detection
The ECG is used to provide the reference signal to frame the second
heart sound. The beat signal in the description below means the
part of acoustic signal between two consecutive QRS complexes on
the ECG. Depending on the selected approach, the "beat signal" can
be defined as the Q-Q' (distance between two Q markers) or as the
R-R' (distance between two R markers). In the following description
Q-Q' provides the beat signal. For each beat signal the first heart
signal (S1) is detected and removed. The remaining sounds,
including the second heart sounds and possibly murmurs and the
like, are then used as input.
Referring now to the flow charts of FIGS. 3A and 3B in addition to
FIG. 1, an illustrative embodiment of an approach for detection of
the aortic component A2 and the pulmonary component P2 of the
second hearts sounds will now be described. The illustrative method
supports input signals from the single or multiple sensor(s) 12,
each of them comprised of signals of heart sounds in the frequency
range 30-200 Hz, although this range could be wider without any
changes in the approach. If that range is narrower, however, the
method should be adapted to those limitations.
Sounds related to heart beats are collected at 100 via a sensor(s)
12 and illustratively divided into three sub channels 102, 104 and
106 (or frequency bands). These bands are: Low Frequency (LF, 30-50
Hz), Medium Frequency (MF, 50-150 Hz), and High Frequency (HF,
120-200 Hz).
Each sub-channel is relayed to a "Process Channel" block as in
108.sub.1, 108.sub.2, and 108.sub.3, (these will be described
separately hereinbelow). The process channel block can be based on
a variety of methods including a Chirplet method, Non-linear Energy
Operator (NLEO) method, or any other suitable method capable of
extracting and discriminating A2 and P2 components from second
heart sound S2.
Of note is that the present illustrative embodiment applies the
NLEO method.
The output values of A2 and P2 from the process channel blocks as
in 108.sub.1, 108.sub.2, and 108.sub.3 are analysed. If both
components A2, P2 are clearly detectable in at least one of the sub
channels, these are the values for A2, P2. If both components are
not clearly detectable then the outputs of the process channel
blocks as in 108.sub.1, 108.sub.2, and 108.sub.3 are compared
sub-channel by sub-channel with the output of the process channel
blocks for other sensors (not shown) of the same sub channels at
blocks 110, 112, and 114. In the case at hand, there are
illustratively two sensors (the second sensor not shown) the
outputs of the process blocks of which are thus compared pair
wise.
Illustratively, the comparison is carried out on each frequency
band according to the following set of rules, although it should be
understood that this is an example and not intended to be limiting:
If the output of 108 for both sensors reveals A2 and P2 components
and the positions of A2 and P2 in each sensor output are the same,
then these positions provide the values of A2 and P2; If one of the
outputs of 108 for both sensors reveals A2 and P2 components, but
the other does not, then the positions of these A2 and P2 provide
the values of A2 and P2; If the output of 108 for both sensors
reveals only one A2 or one P2 component then, as it is unknown
whether the component is A2 or P2, then the value of A2 is the
position of the first component and the value of P2 the position of
the second component. If the output of 108 for one of the sensors
reveals both A2 and P2 components while the output of 108 for the
other sensor reveals only one (A2 or P2) component, then the
readings for both sensors are combined (superimposed). If the
result reveals only two components (A2 and P2) then the positions
of these A2 and P2 provide the values of A2 and P2; If the result
still reveals three components (where one or two of the results are
A2 and/or P2 and the remainder the result of biological noise),
then the readings are combined (superimposed) and the two
components with the greatest FWE are selected as A2 and P2, the
positions of these A2 and P2 provide the values of A2 and P2. If
the output of 108 for both sensors reveals A2 and P2 components but
the positions of A2 and P2 are different, then: If the Splitting
Interval (SI) of both sensors is less than 10 ms then the value of
A2 is the position of A2 and the value of P2 is the position of P2
as determined via one of the sensors; If at least one of the SI
from first or second sensor is greater than 10 ms, all components
(A2 and P2) within 10 ms are merged. If only one component results,
then the value of both A2 and P2 is the position of this one
component and resulting SI is equal to zero; If two components
result, then the value of A2 is the position of the first component
and the value of P2 the position of the second component; If three
components result, then the values of A2 and P2 are the positions
of the two components with the greatest FWE; and If four components
result, then the values of A2 and P2 are the positions of A2 and P2
from the sensor where the amplitude of components FWE is greater
than that of the other sensor.
A similar approach is used in the case of multiple sensors.
The SI for each sub-channel, including combined channels, is also
calculated.
The A2 and P2 components in the LF, MF, and HF sub-channels have
small variations in positioning because of different frequency
content. As a result, at block 116, heuristic rules are used to
correct those deviations and produce A2 and P2 single values from
the combination of A2 and P2 from all sub-channels (LF, MF, HF) as
well as any combined values which may have been generated. An
illustrative example of the heuristic rules applied at block 116 is
as follows: If no values for both A2 and P2 are available in the MF
and HF sub-channels and the SI of the LF channel>120 msec, then
discard the SI of the LF channel; If values for both A2 and P2 are
available in the LF and HF sub-channels and the SI of the LF
channel>1.4*SI of the HF channel, then discard the SI of the LF
channel; If values for both A2 and P2 are available in the LF and
MF sub-channels, and the SI of the LF channel>1.4*the SI of the
MF channel, then the SI of the LF channel=1.4*the SI of the MF
channel; and If values for both A2 and P2 are available in the MF
and HF sub-channels, and the SI of the MF channel<1.4*the SI of
the HF channel, then the SI of the HF channel=(1/1.4)*the SI of the
MF channel.
Referring now to FIG. 3B, the values of A2, P2 and SI for the
current beat are stored at blocks 124, 126 and 128. Illustratively,
values of A2, P2 and SI calculated for beats during the previous
minute are retained.
At the same time consistency of solution and signal-to-noise ratio
(SNR) for each sub-channel is estimated and stored in separate
lists. In this regard, for each sub-range the SNR is estimated.
Consistency indicates the percentage of beats not rejected due to
high noise. Illustratively, in order to determine the SNR, the S2
sound is first detected as well as the precise position of the
start and end of S2. The signal component (S) is calculated as the
energy between the start and end of S2, divided by the duration of
S2 (in msec). The noise component (N) is calculated as the energy
within 50 msec segment before the start of S2 added to the energy
within 50 msec segment after the end of S2 divided by 100 msec. The
resulting signal-to-noise ratio is calculated as SNR=S/R.
After all beats within the time averaging interval (in the case at
hand illustratively 1 minute) have been processed in the above
manner, a series of values of A2, P2 and SI are ready for
statistical validation. At a first step of the validation process
the distributions of A2 and P2 are estimated and a threshold
location in time from the start of S2 value T calculated using the
bias criterion. Typically between 50-200 beats are present during a
one minute sampling interval. Histograms are used in order to
provide an estimation of the distributions. The distribution law of
SI is used for additional control of the T value in the case of
multi-peak distribution of A2 or P2.
At block 130, any values of A2 which are located at a time greater
than T from the start of S2 and values of P2 located at a time of
less than time T from the Start of S2 are discarded from the stored
values. The SI values are then recalculated at block 132 using only
those A2 and P2 values which still have pairs.
At blocks 134, 136 and 138 the central peaks on the A2, P2 and SI
histograms are estimated using a two-iteration method. During a
first iteration the central peak of each histogram is identified.
During a second iteration, 20% of the input values, those which are
the most distant from each central peak are removed. The histogram
is rebuilt using only the remaining input values. Then at block 140
the value SI'=P2-A2 is calculated.
At block 142, SI' is compared with the peak value of SI calculated
at block 138. If the difference between SI and SI' is less than 1%
of the average beat duration, the mean value of SI and SI' is
produced as the final output value for SI. If the difference
between SI and SI' is greater than 1% of the average beat duration,
the values of SI, SI' having a higher consistency value, as
previously calculated at blocks 144, 146 provides the final output
value.
Referring back to FIG. 3A, as stated hereinabove, the process
channel block 108 can be based on a variety of methods including a
Chirplet method, NLEO method, or any other suitable method capable
of extracting and discriminating A2 and P2 components from second
heart sound S2. Illustratively, the NLEO method is described and
comprises the following processing steps. Referring to block
108.sub.2, The Signal to Noise Ratio (SNR) is determined at block
148. The NLEO method is described in "Adaptive Segmentation of
Electroencephalographic Data Using a Nonlinear Energy Operator" by
Agarwal, et al., Proceedings IEEE ISCAS '99, Orlando, Fla., 1999,
which is incorporated herein by reference.
At decision block 150, if the SNR is below a predetermined value
(illustratively 1.5), the current beat in the channel being
processed is discarded and no further processing steps carried out.
Alternatively, if the SNR is above a predetermined value the NLEO
function is calculated at block 154 using the current beat's
signal.
In this regard, the NLEO or any other individual implementation of
FWE or any other individual implementation of the general family of
Autocorrelators may be used.
NLEO is a manipulation of digital signal described in the general
case by: .PSI.[n]=x(n-l)x(n-m)-x(n-p)x(n-q) for l+m=p+q (1)
One of NLEO's special properties is the ability to compactly
describe the notion of a Frequency Weighted Energy (FWE), which is
different from the mean-square energy as it reflects both the
amplitude as well as the frequency content of a signal. For the
special case where l+p=q+m, l.noteq.p and q.noteq.m., given an
input of additive white Gaussian noise (AWGN) the expected value of
NLEO output is zero. Thus it has the ability to suppress noise. If
we consider the case of an amplitude modulated short duration
sinusoidal burst in the presence of random noise and structured
sinusoidal interference (as in the case of the aortic and the
pulmonary components of the S2 sound in the midst of noise), it is
anticipated that the NLEO output will enhance FWE of each of these
components while suppressing AWGN interference and provide a
constant baseline for sinusoidal interference. The time-varying
nature of amplitude (Gaussian) and chirping of the dominant rhythm
will modulate the NLEO output and produce a detectable burst
corresponding to each component in contrast to background clutter.
It will then be possible to apply detection strategies on the NLEO
output with S2 sound input.
Illustratively, NLEO with parameters l=2, m=1, p=3, q=4 was
applied.
Once the NLEO function is calculated, at block 156 the highest peak
(maximum of NLEO output for given beat signal) is determined and
those peaks having values of less than 0.05 of highest peak value
are removed. In this regard, 0.05 provides good results, although
other values may also provide adequate results. If more than two
peaks remain, the A2 and P2 candidates are identified at block 158.
If only one peak is detected, then this is passed to the output and
determined as A2 or P2 according to the procedure described
hereinabove at paragraph 18.
Finally, at block 160 the values of A2 and P2 are validated using
list of heuristic rules. An illustrative example of such rules are:
If the time interval between A2 and P2 on NLEO is greater than 100
msec, the component with lower FWE is considered invalid; and if
the time interval between A2 and P2 on NLEO is less than 10 msec,
the component having a lower FWE is considered invalid.
Although the present invention has been described hereinabove by
way of an illustrative embodiment thereof, this embodiment can be
modified at will, within the scope of the present invention,
without departing from the spirit and nature of the subject of the
present invention.
* * * * *